This invention relates to new radiation-sensitive positive photoresist compositions. In the production of an image using a positive photoresist, the portions of the resist that are exposed to actinic radiation become more soluble in or more easily removed by a developer compared to the unexposed regions. Removal of the exposed regions by means of a developer, leaving the unexposed areas intact, results in the production of a positive image.
Generally commercially available positive photoresist compositions contain, in addition to an organic film-forming material which is usually a phenol-formaldehyde (novolac) resin, an O-naphthoquinone diazide compound which decomposes on exposure to actinic radiation to produce an indene carboxylic acid with the evolution of nitrogen. Before exposure, the O-naphthoquinone diazide acts as a dissolution inhibitor for the organic film forming material. Upon exposure, however, the diazide is decomposed and completely loses its ability to act as an inhibitor and, in fact, becomes a dissolution enhancer. Thus, the dissolution rate of the exposed area becomes greater compared to that of the non-exposed areas. However, there are several problems associated with the use of O-naphthoquinone diazides, foremost among these is their thermal instability. The diazides decompose at moderately elevated temperatures, even with protection from actinic radiation. This potential to decompose at moderately elevated temperatures renders the production of a quality image at high processing temperatures impossible. In addition, relatively large quantities of these diazides are required to make them useful as dissolution inhibitors. Also, because they absorb very strongly in the deep ultraviolet region, photoresist compositions employing these diazides are relatively opaque to deep ultraviolet radiation and thus cannot be developed fully. This problem results because the radiation cannot penetrate deeply into the photoresist coating.
Attempts to correct the problems associated with the diazonaphthoquinone inhibitors led to the production of 5-diazo Meldrum's acid and derivatives, as disclosed in U.S. Pat. No. 4,339,522. Although these materials, specifically designed for the deep ultraviolet region, were much more sensitive to the deep UV compared to the classical diazonaphthoquinone, these Meldrum's acid diazo compounds were not usable at high processing temperatures due to the fact that these compounds were evolved (sublimed) from the matrix resin to a significant extent at a relatively low temperature. Attempts to modify the structure of Meldrum'sdiazo led to several other problems, including insolubility in common casting solvents, as well as the sublimation problem described above. This information is reported in IEEE Trans. Electron Dev., Vol. ED-28(11), 1300 (1981).
European Patent Application No. 241,423 to Demmer discloses positive working photoresist compositions using light sensitive oxime sulfonate compounds. Sulfonate oximes generally have a lower heat stability and are capable of crosslinking polymers at elevated temperatures.
A photoresist composition comprised of o-nitrobenzyl cholate inhibitor in poly(methylmethacrylate-co-methacrylic acid) (P(MMA-MAA) resin, disclosed in U.S. Pat. No. 4,666,820 issued May 19, 1987, (see also U.S. Pat. No. 4,400,461) was reported to be photoimageable employing deep ultraviolet radiation. Specifically, this composition was reported to yield surprisingly high contrasts. However, the mechanism reported for this photoreaction is not a completely conventional one; i.e. the mechanism involves inhibitor-assisted breakdown of the base-soluble P(MMA-MAA) resin in the exposed areas relative to the unexposed areas. The conventional mechanism, i.e. breakdown of the base-insoluble inhibitor into base-soluble fragment(s), leaving the resin binder entirely in tact, to produce a dissolution rate enhancement, is operating only to a very small extent. This information can be found in the Journal of Polymer Science: Polymer Chem. Ed., 21, 1975 (1983). The propensity for poly(methylmethacrylate) (PMMA) resin to degrade in the presence of deep ultraviolet radiation or free radicals is well known in the art. Other conventional base-soluble resins (e.g. novolac or poly(hydroxystryene)) do not display this type of behavior. It would therefore be highly unlikely that a photoresist comprised of o-nitrobenzyl cholate inhibitor in novolac or poly(hydroxystyrene) resin could demonstrate the surprisingly high contrasts observed with P(MMA-MAA).
PMMA, compared to novolac or poly(hydroxystyrene), is also an undesirable resin for use in photoresists due to its extremely low plasma etch resistance.
In addition, the o-nitrobenzyl cholate/P(MMA-MAA) resist has been reported to possess poor dark field erosion resistance and relatively low sensitivity to 248 nm narrow band radiation. No high resolution patterns (e.g.&lt;1.0 micron) were obtainable under these conditions. This information is available in J. Vac. Sci. Technol. B., 5, 396 (1987).
It had not been previously known that oxime carboxylate esters, when present with a base-soluble organic film-forming material, could suppress the dissolution rate of this film-forming material and that, after exposure to deep ultraviolet radiation, the oxime carboxylate ester would lose its ability to become a dissolution rate inhibitor and become a dissolution rate enhancer.
It is the object of this invention to provide new, positive-working photoresist compositions which possess high sensitivity to the deep ultraviolet region, producing quality, aqueous base developable images after exposure in a pre-determined pattern. In addition, the dissolution inhibitor of this invention is highly stable toward thermal degradation. This advantage allows for the production of high quality images at relatively high processing temperatures. Still another advantage of the present invention over the coventional O-naphthoquinone diazide-type systems is that the oxime carboxylate ester inhibitor compounds have little or no absorption of light in the conventional UV range (above 300 nm) and hence require no optical filters to obtain the higher resolution images available employing only shorter wavelength (deep UV) radiation.